Recently, a new class of .beta.-lactam antibiotic compounds having a sulfo group in 1-position have been found to be recoverable from nature sources and reported [Nature 289, 590 (1981), 291, 489 (1981)]. Synthesis of compounds related thereto has also been reported (e.g. EP-A1-0021678 and EP-A1-0048953). The latter literature teaches a method of synthesizing 2-azetidinone-1-sulfonic acid compounds having a substituent in 4-position but the method involves a long series of reaction steps and no expedient process for the purpose has been reported.